The present invention relates to data transmission and in particular to an improved signal structure for transmitting a secondary channel in a quadrature amplitude modulation (QAM) constellation.
In U.S. Pat. No. 3,887,768 issued June 3, 1975 to Formey, Jr. et al. for SIGNAL STRUCTURES FOR DOUBLE SIDE BAND QUADRATURE CARRIER MODULATION, a data transmission system is described in which the signal points are mapped in the complex plane on concentric rings. The signal point of each ring is rotated by 45.degree. from those of the next adjacent ring and no point is allowed at the origin. Such signal constellations are widely used in data transmission systems.
To increase the data carrying capabilities of such transmissions systems, it has heretofore been proposed to introduce a secondary channel to the primary channel QAM signal. The problem is to introduce the secondary channel with no degradation of the primary channel and with no substantial variation in the power requirements of the transmission system when the secondary channel is utilized. Heretofore it is has been proposed to introduce the secondary channel by shifting the angular position of the data points. Thus, when the data points assume a first position, a binary "1" is transmitted for the secondary channel and when the data points assume a secondary position, angularly shifted with respect to the first position, a binary "0" is transmitted for the secondary channel. The problem with this procedure is that it significantly reduces or degrades the performance of the primary QAM channel.
In view of the above, it is the principal object of the present invention to provide an improved system for the transmission of secondary channel signalling in a QAM constellation.
A further object of the present invention is to provide such a system which introduces virtually no degradation to the primary channel.
A still further object is to provide such a system in which the secondary channel may be added without any additional power requirements over that of the primary QAM channel.
A still further object is to provide such a system wherein the secondary channel may be asynchronous.
Still other objects and advantages will be self-evident from the following.